Particle counting apparatus

ABSTRACT

A particle detection apparatus for determining the number and size of particles present in a plurality of particles dispersed over a field uses an oscillating mirror and movement of the field transversely to the mirror movement to optically scan the field with a succession of scan cycles. Particle indicating signals are produced each time a particle scanned satisfies intensity and size thresholds. Only one particle indicating signal for a particle is used to produce a count signal. Particle indicating signals developed by scans other than the scan from which a count signal for the particle is developed are suppressed. This is accomplished by circuitry including a coincidence circuit and a memory provided by a shift register circuit controlled by a selected number of clock pulses provided for each scan cycle. A feedback circuit is connected to respond to the initiation of a particle indicating signal to assure continuation of the particle indicating signal for the remainder of the time such scan intercepts the particle. The feedback circuit is also connected to respond to the output of the shift register circuit to improve the operation of the circuitry providing for suppressing the signals developed by scans of a particle in excess of the one needed for a count signal. A meter reading is provided which is indicative of the number of particles intercepted when scanning with the field in a selected position to enable the operator to set the intensity threshold correctly.

United States Patent [191 Schoon Feb. 18,1975

PARTICLE COUNTING APPARATUS Inventor: David J. Schoon, Saint Croix, Minn.

[22] Filed: Nov. 1, 1972 [21] Appl. No.: 302,810

[52] U.S. Cl 235/92 PC, 235/92 SH, 235/92 R, 250/222 PC, 356/102 [51] Int. Cl. G06m 11/04 [58] Field of Search 250/222 PC; 356/102; 235/92 PC, 92 SH, 92 CA; 350/6'; 340 146.3 F

[56] References Cited UNITED STATES PATENTS 2,803,406 8/1957 Nuttall 235/92 PC 2,948,470 8/1960 Berkley et al.... 235/92 PC 3,088,036 4/1963 Hobbs 235/92 PC 3,178,688 4/1965 Hill et al. 235/92 PC 3,437,394 4/1969 Hatcher et al. 350/6 3,557,352 1/1971 Hogg et al. 235 92 PC Primary ExaminerGareth D. Shaw Assistant Examiner.loseph M. Thesz, Jr. Attorney, Agent,or Firm'Alexander, Sell, Steldt & Delahunt [5 7] ABSTRACT A particle detection apparatus for determining the number and size of particles present in a plurality of particles dispersed over a field uses an oscillating mirror and movement of the field transversely to the mirror movement to optically scan the field with a succession of scan cycles. Particle indicating signals are produced each time a particle scanned satisfies intensity and size thresholds. Only one particle indicating signal for a particle is used to produce a count signal. Particle indicating signals developed by scans other than the scan from which a count signal for the particle is developed are suppressed. This is accomplished by circuitry including a coincidence circuit and a memory provided by a shift register circuit controlled by a selected number of clock pulses provided for each scan cycle. A feedback circuit is connected to respond to the initiation of a particle indicating signal to assure continuation of the particle indicating signal for the remainder of the time such scan intercepts the parti' cle. The feedback circuit is also connected to respond to the output of the shift register circuit to improve the'operation of the circuitry providing for suppressing the signals developed by scans of a particle in excess of the one needed for a count signal. A meter reading is provided which is indicative of the number of particles intercepted when scanning with the field in a selected position to enable the operator to set the intensity threshold correctly.

26 Claims, 7 Drawing Figures PATENHUFEBWQYS 3,867. 613

' I SHEET w 4 (F/GJ) (F/6.4) I 30* 1 PARTICLE COUNTING APPARATUS Field of the Invention The invention presented herein relates to particle detection apparatus using a line by line scan of a field for counting particles in the field which are at least of a predetermined size with provision made to compare a scan line with a prior scan line to prevent multiple counts for a single particle.

BACKGROUND OF THE INVENTION The fields of biology, medicine, photogrammery, metallurgy, pollution control, pharmaceutical manufacture and many others require apparatus for automatically sizing and counting various particles or objects.

A few instruments are presently available which provide such counting and sizing with varying degrees of accuracy. Those instruments capable of providing a fair degree of accuracy are very expensive. In order that the use of apparatus of this type can expand, there is need for an accurate and flexible particle measuring apparatus that can be sold for a more reasonable amount. Presently known apparatus use designs which require costly major components.

DESCRIPTION OF TI-IE'PRIOR ART Automatic examination of particles randomly distributed over a field has been accomplished by scanning the field by a light beam. For example, a light spot scan developed on the screen of a cathode ray tube which is imaged upon the field has been used. The intensity of light passed through or reflected by the field which varies in accordance with the particles present in the field is measured by a photomultiplier tube. A vidicon tube has also been used by focusing the field carrying particles onthe screen of the tube which is then scanned by an electron beam. However, any scanning arrangement using cathode ray tubes or vidicon tubes is expensive, bulky and presents maintenance problems which can be handled only by specially trained personnel.

Some particles overlap several lines of scan causing a change in the output of the photomultiplier for each interception. It was recognized that some form of line to line memory was needed so the signal output for a given line scan could be compared with a representation of the signals generated one line previously with provision made to suppress signals when correspondence of the signals was found to prevent multiple counts of a particle. Magnetic drum and tape recording arrangements have been used as well as delay lines to provide a memory for counting apparatus. The mag netic recording approach is very expensive, bulky and presents maintenance problems. Delay lines are not adjustable which requires the sanning frequency to be precisely matched to the fixed time delay provided by a delay line. In addition, delay lines are temperature sensitive, provide poor resolution and are expensive.

Accurate intensity threshold adjustment of automatic counting apparatus is essential to prevent erroneous counts due to the apparent bridging between particles that can occur when the particles are not clearly defined. Prior art counting apparatus includes the use of a television type display developed by scanning a field which the operator visually compares with the actual field to obtain an adjustment of the threshold levels.

Such an arrangement for making the threshold adjustments is expensive and requires a great deal of judgment on the part of the operator. Another approach to adjustment of the threshold levels requires a physical count of the particles by the operator which is compared with the apparatus count. This is time consuming and subject to operator error.

SUMMARY OF THE INVENTION The invention presented herein uses an oscillating mirror scanning arrangement with'a photomultiplier detector which is considerably less expensive and presents less complex maintenance problems than the cathode ray tube and vidicon tube scanning systems employed in prior known automatic particle counting apparatus. An oscillating mirror positioned in an optical path scans back and forth across the field containing the particles to be detected while the field is moved transversely to the mirror scan to provide a sinusoidal raster type scan of the field. The light intensity from the scanned field varies in accordance with the particles in the field and is detected by a photomultiplier tube, the outputof which is processed via minimum intensity and size threshold circuits to provide particle indicating signals.

The use of delay lines or magnetic recording arrangements to provide the scan to scan memory needed for an automatic particle counting apparatus has also been avoided by this invention. A shift register circuit is used to provide the memory. In addition to providing a less costly memory with excellent resolution, the use of a shift register circuit does not place any limitation on the scan frequency used. A synchronizing pulse obtained from the oscillating mirror indicative of the beginning of each scan cycle is applied to a clock pulse generator causing it to initiate a selected number of clock pulses for each scan cycle. The shift register circuit uses the selected number of clock pulses for a cycle of its operation. The clock pulses control the response of the shift'register circuit to particle indicating signals applied to its input to provide a memory for use in comparing the particle indicating signals produced during one scan with the particle indicating signals present during the preceding scan so only one of all the signals generated for a given particle detected will be used to provide a count signal. The shift register circuit receives each particle indicating signal and for each one received provides a particle indicating signal at its output after the shift register circuit has received the number of clock pulses required for one cycle of operation. A particle indicating signal at the output of the shift register circuit during a current scan indicates a particle indicating signal was received by the shift register circuit the selected number of clock pulses earlier during the preceding scan. A signal coincidence circuit, which presents a count signal in response to a first signal unless a second signal is received by the coincidence circuitry during the first signal, is connected to receive the particle indicating signals obtained from I processing the output of the photomultiplier as one of the first and second signals with the particle indicating signals from the shift register circuit providing the other of the first and second signals.

A synchronizing pulse developed from the oscillating mirror is supplied to the clock pulse generator at the beginning of each scan cycle so long term drift of the scanning frequency as determined by the oscillating mirror and/or the clock pulse generator frequency is permissible without introducing any erroneous counts. This is a most desirable feature since absolute stability of either is difficult and costly to obtain.

The rate at which the clock pulses from the clock pulse generator are provided can be varied. This permits the rate to be adjusted so the selected number of clock pulses are completed within the scan in one direction or just prior to the completion of the return scan.

The oscillating mirror scanning arrangement produces a sinusoidal scan of the field providing a rate of scan which is inherently nonlinear. Thisinvention provides circuitry to correct for the varying rate of scan when a sinusoidal scan is used.

The signals obtained from the photomultiplier detector contain considerable noise which varies as a scan is made across a particle. This noise is due to such factors as changnes in the light source used for scanning, the inherent noise of the photomultiplier, variations in the opacity of the particle being scanned, as well as variations in the opacity of the field holder. This noise can vary during a scan across a particle and also from one scan to the next and can give rise to erratic triggering of the threshold circuits used for establishing the minimum intensity and minimum size required for detection of a scanned particle. The invention presented herein prevents erratic operation due to such noise by providing a feedback signal to the minimum intensity and size threshold circuits once the photomulitplier output signal satisfies the minimum intensity and minimum size thresholds. Once the initial intensity and size requirements for a particle have been met, information obtained in the prior scan of an object is combined with the current scan and applied to the feedback circuit. Information from a prior scan provides correction for the variation in'the signal from scan to scan and is provided by applying the shift register circuit output to the feedback circuit with the shift register circuit advanced one clock pulse for each scanning cycle. Advancing the shift registercauses the shift register circuit to provide a signal to the feedback circuit earlier in a scan that it functioned in the preceding similar scan. The shift register circuit is then more likely to have an input signal applied to it for each intercepting scan in a given direction'following the first particle indicating signal produced by ascan of a particle made in such direction so that the necessary particle indicating signals from the shift register circuit are provided to prevent the production of more than one count signal for a given particle.

The invention also provides circuitry to enable the operator of the apparatus to easily adjust the intensity threshold correctly without the use of a television type display or physical count of the particles. Since the scanning arrangement utilized in this invention permits the field holder to be physically moved, the operator can position the field so the oscillating mirror will be scanning a representative portion of the field. A meter reading provides a visual measure of the number of interceptions producing a particle indicating signal during scan of the field in the selected position as the intensity threshold adjustment is varied to facilitate making the adjustment correctly.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the invention,

reference should be made to the accompanying drawings, wherein like parts in each of the several figures are identified by the same reference characters, and wherein FIG. 1 is a blockdiagram of the particle detection apparatus embodying the invention;

FIG. 2 is a representation of the raster type of sinusoidal scan used in connection with apparatus of FIG. 1;

FIG. 3 consists of a number of signal patterns for use in describing the operation of the apparatus of FIG. 1; and

FIG. 3A consists of a number of signal patterns for use in describing the operation of the apparatus of FIG. 1 when modified slightly;

FIGS. 4, 5 and 6 are exemplary circuits for use in an apparatus as shown in FIG. 1.

DETAILED DESCRIPTION The particle detection apparatus of FIG. 1 includes a scanning portion 10 which optically scans a field 12 to detect particles that are at least equal to a predetermined size and produces a particle indicating signal when such a particle is scanned. The particle indicating signals are applied to a memory provided by a shift register circuit 35 and to signal coincidence circuitry 48 which is also connected to theouput of the shift register circuit. A clock pulse generator 11 connected to the shift register circuit provides clock pulses for determining'when the shift register circuit will respond to particle indicating signals applied via conductor 41 to the input of the shift register circuit.

The scanning portion 10 includes the optical scanner l4 and a light responsive device 15, which may be a photomultiplier tube, for providing signals when particles in the field 12 are intercepted by a scan plus signal processing circuitry which includes amplifier 16, minimum intensity circuit 17, minimum size circuit 18, pulse stretcher circuit 19 and feedback circuit 20 for providing particle indicating signals when the signals from photomultiplier l5 satisfy operator selected intensity and size thresholds. The optical scanner 14 includes an oscillating mirror assembly 21, a mirror control circuit 22, a light source 23, a Fresnel or condensing lens 24, focusing lens 25, pin-hole 26, and motor 27. The mirror assembly 21 includes a mirror 93. While the optical path shown in FIG. 1 for scanner 14 uses light transmitted by the field 12, it is apparent to those skilled in the art that light reflected from the field 12 could be used.

The mirror control circuit 22 is electrically connected to the oscillating mirror assembly 21 via conductors 28 and 29. Conductor 28 provides a signal from the mirror assembly 21 to the mirrorcontrol circuit 22 indicative of the mirrors position while conductor 29 serves to apply a drive signal to the mirror assembly 21 from the'mirror control circuit 22. The mirror control circuit 22 also provides a synchronizing pulse to the clock pulse generator 11 at the beginning of each cycle of the mirror 93 via the conducting path 30.

The light from light source 23 passes through the condensing or Fresnel lens 24 and the field 12 containing the particles to be measured to the mirror 93 where it is reflected for passage to the focusing lens 25, thence via the pin-hole 26 to the photomultiplier 15.

At the start of a scan of the field 12, the motor 27 drives the field 12 to the right as viewed in FIG. 1, while the mirror 93 oscillates to produce a scanning locus that is transverse to the movement of the field 12 causing the field 12 to be scanned by a sinusoidal scan of the type illustrated in FIG. 2. The scanning to the right and left, as shown in FIG. 2, is due to the movement of mirror 93, while the movement of the field 12 provides vertical coverage of the field. The scan lines are much closer together than is shown in the drawing for FIG. 2 which is not drawn to scale. One scan cycle includes a scan from one side to the other and return. The degree of resolution for the vertical portion of the scan, as shown in FIG. 2, is determined by the speed with which the field 12 is moved, while resolution provided by the side to side scanning, is a function of the number of pulses provided by the clock pulse generator 11 and the portion of the scan cycle during which the pulses are provided. A particle 1 is shown in FIG. 2 to illustrate how a particle may be intercepted by more than one scan.

The clock pulse generator 11 provides a selected number of clock pulses following the receipt of each synchronizing pulse from the mirror control circuit 22. The clock pulse generator 11 may include an oscillator 31, a divider 32 and a flip-flop circuit 33. The output of the oscillator 31 is applied to the divider 32 which, after receiving the selected number of pulses to be provided by the pulse generator 11, supplies a set signal to the flip-flop 33 causing the flip-flop circuit to provide a signal to the oscillator 31 via conductor 34 which 'is effective to inhibit further operation of the oscillator. The synchronizing pulse from the mirror control circuit 22 for the clock pulse generator 11 is applied via conductor 30 to the flip-flop 33 and serves to reset it to remove the inhibiting signal supplied to osciallator 31, causing it to again provide the selected number of clock pulses.

Theselected number of clock pulses provided by the generator 11 is equal to the number of clock pulses required by the shift register circuit 35 to have its response to a given input signal appear at its output. The shift register circuit receives clock pulses from the clock pulse generator 11 via conductor 36 NAND circuit 37 and conductor 38. These clock pulses are also applied to a pulse stretcher circuit 19 from conductor 38. The signal pattern 39 in FIG. 3 represents the clock pulses as they appear at the output of the NAND circuit 37. The clock pulses supplied to the divider 32 are also supplied to the pulse stretcher circuit 19 via conductor 13 and are represented by the signal pattern 40 in FIG. 3.

Adjustment of the frequency of the scanning cycles and/or the rate at which the clock pulses are produced makes it possible to establish the point in a scan cycle when the last of the selected number of clock pulses for a scan cycle is produced. In the apparatus of FIG. 1 the pulse rate or frequency of the pulse generator 11 is adjustable and is set so the selected pulse number of clock pulses, which begin with the synchronizing pulse provided at the beginning of each cycle of the mirror 93, are completed immediately prior to the completion of the first half of scan cycle. That portion of the scan cycle during which clock pulses are provided will hereinafter be referred to as an operative scan.

As the plurality of particles dispersed over the field 12 is scanned, the output of photomultiplier 15 changes with the variations in light intensity at the photomultiplier 15 due to interception of the particles bythe scanning process. The output of the photomultiplier is applied to the amplifier 16 to provide a signal level that can be processed by circuitry 17-20.

Signals at the output of amplifier 16 due to the interception of a particle must satisfy an intensity threshold, i.e., the amplitude must be at least equal to the minimum level determined by the minimum intensity circuit 17. The minimum intensity setting is controlled by the operator of the apparatus. If the signal satisfies the established intensity threshold, it is then applied to the minimum size circuit 18 to determine whether it is of a duration sufficient to satisfy the minimum size threshold established by that circuit. The minimum size setting is also controlled by the operator. If the minimum size threshold is satisfied, the circuit 18 provides a signal which indicates that all threshold requirements have been satisfied. Such a signal will hereinafter be re ferred to as a particle indicating signal. The particle in dicating signal has a duration indicative of the extent to which the intercepting scan of the particle continues beyond the intercept length required to satisfy the minimum size threshold.

It is apparent that a particle indicating signal can have a very short duration when produced in response to a scan of a particle which only continues for a very short time beyond the duration needed to satisfy the size threshold as set by the circuit 18. Before the particle indicating signals are used further for counting particles they are applied to the pulse stretcher circuit 19 which serves to extend the duration of each particle indicating signal received from the minimum size circuit 18 during an operative scan. The stretched particle indicating signal appears at the output conductor 41 which connects with the input of shift register circuit 35. The shift register circuit 35 is operated by a signal applied to its input if such signal is present when a positive-going clock pulse is received by the shift register circuit. A particle indicating signal must be present during at least one of the clock pulses in order to have the shift register circuit 35 respond to the pulse indicating signal. The pulse stretching action of circuit 19 is therefore quite important since it will assure the signal coincidence needed for proper operation of the shift register circut 35 in response to an input signal from circuit 19.

The pulse stretching operation of circuit 19 can be best understood by considering the signal patterns shown in FIG. 3. The signal patterns are representative of the operation of a counting apparatus according to this invention using the pulse stretcher circuit 19 shown in FIG. 4 which includes a signal inverter 65 and two D-type flip-flop circuits 66 and 67. The flip-flop circuits can be those provided by a commercially available dual unit identified as an SN747N integrated circuit.The clear and D inputs for flip-flop 66 are connected to the output of the inverter 65 which has its input 68 connected to the output of the minimum size circuit 18. The clear and D inputs for flip-flop 67 connect with the Q output of flip-flop 66. The preset terminals for each flip-flop connect with a positive 2.5 volt source (not shown). The flip-flop circuits 66 and 67 respond to low level clear and preset inputs and to positive going clock pulses. Clock pulses are appled to flipflops 66 and 67 via conductors 38 and 13, respectively.

Referring to FIG. 3, the signal pattern 42 is representative of an output signal for the photomultiplier 15. The two positive portions indicate the interception by a scan of two particles. The signal pattern 42 causes the digital type signal pattern 43 to be presented at the output of the minimum size circuit 18. The clock pulses appearing on conductor 13 are represented by the signal pattern 40, while the signal pattern 39 represents the clock pulses which appear on conductor 38. The response at the Q output of flip-flop 66 to the signal pattern 43 is represented by the signal pattern 44. The response at the output of flip-flop 67 to the signal pattern 43 is represented by the signal pattern 45. The effective data input shifted by shift register circuit 35 in response to the signal pattern 45 from pulse stretcher circuit 19 is indicated by the signal pattern 46. The inverse of signal 45 is produced at the O output of the flip-flop 67 to which conductor 47 is connected and is indicated by the signal pattern 45.

With the pulse stretcher circuit 19 described, a high signal at the input of inverter 65 is presented as a low level to the clear input of flip-flop 66 which is immediately transferred as a low signal at Q of flip-flop 66 with such signal remaining until the input signal to inverter 65 becomes low and a positive-going clock pulse transition is received from conductor 38. Thus, referring to the first positive pulse in the signal pattern 43, which is applied to the input of the inverter 65, the signal 44 at Q of flip-flop 66 becomes low and remains so until the input to inverter 65 per signal pattern 43 becomes low and a positive going clock pulse transition is received by flip-flop 66 via conductor 38 per signal pattern 39. The low signal at Q of flip-flop 66 causes the Q output of flip-flop 67 (signal pattern 45) to be low and remain so until the signal at Q of flip-flop 66 is high and a positive going clock pulse transition per signal pattern 40 is applied to the flip-flop 77 from the clock pulse generator via conductor 13. The first positive going pulse per signal pattern 43 is thus stretched or increased in duration as shown by the signal pattern 45 and acts as a data input to the shift register circuit 35 since it is present when the second clock pulse per signal pattern 39 is presented to the shift register circuit 35. As can be seen, this coincidence for operation of the shift register circuit 35 would not have occurred had the signal from the minimum size circuit 18 per signal pattern 43 not been increased in duration. The same is true for the second pulse in the signal pattern 43.

It should be noted that the degree to which a particle indicating signal is increased in duration varies since the incnrease is dependent on its duration and the time of occurrence in relation to the clock pulses. A circuit providing a fixed delay is not as desirable since erroneous counts will be produced under some circumstances. The circuit 19 described does have a limit as to the delay that can be introduced. The greatest delay possible will provide an output signal from the pulse stretcher circuit 19 having a duration of one and onehalf clock pulse cycles.

A particle in a field being scanned may be scanned more than once giving rise to more than one particle indicating signal from the pulse stretcher circuit 19 for a given particle. It is desirable that only one such signal be used to count the particle. The pulse stretcher 19 on the first scann scan a particle resulting in a particle indicating signal provides information indicating the particle satisfies the threshold requirements. At any point in an operative scan, the shift register circuit 35 provides an output signal which is indicative of the input signal it received from the pulse stretcher circuit 19 during the corresponding portion of the preceding operative scan. Accordingly, the output of the shift register circuit 35 at the time the firs particle indicating signal is produced for a given particle will indicate a particle indicating signal was not produced for the given particle during the preceding operative scan. The shift register circuit 35 during the subsequent operative scan of the given particle will provide an output signal indicating that a particle indicating signal was produced by the preceding scan of the given particle. A signal at the output of the shift register circuit 35 providing such information will hereinafter be referred to as a shift register circuit particle indicating signal.

Signal coincidence circuitry 48 provides the logic needed to use the output from the pulse stretcher circuit l9 and the shift register circuit 35 for determining whether a count signal should be produced. The circuitry 48 includes a leading edge detector 49, a trailing edge detector 50, a flip-flop circuit 51 and a NOR circuit 52. The circuit details for circuitry 48 is shown in FIG. 6. A positive signal presented on conductor 47 will cause a count signal to be presented at the output of NOR circuit 52 provided a positive or high signal is not received at the reset input 54 of flip-flop 51 prior to the termination of the positive signal presented on conductor 47. Accordingly, a particle indicating signal is received from pulse stretcher circuit 19 via conductor 47 and provides a count signal unless a particle indicating signal from the output of the shift register circuit 35 is received by the flip-flop 51 before the particle indicating signal from pulse stretcher circuit 19 is terminated. The output of the shift register circuit 35 is presented via the conducting path 53 to the reset input 54 of the flip-flop 51, while the output from the pulse stretcher circuit 19 is applied via conductor 47 to the input for the leading edge detector 39 and the trailing edge detector 50. The output from the leading edge detector 49 is connected to the set input 55 of the flipflop 51. The output for the flip-flop 51 is applied to input 56 of the NOR circuit 52 with another input 57 being supplied from the output of the trailing edge detector.

A particle indicating signal provided by the pulse stretcher 19 to the leading edge detector 49 and trailing edge detector 50 is shown by the signal pattern 4 5. in response thereto, the leading edge detector 49, as shown by signal pattern 5 of FIG. 3, provides a sharp positive going pulse at its output corresponding to the leading edge of the signal which causes the flip-flop circuit 51 to be set to present a low signal to the input 56 of the NOR circuit 52. If the preceding operative scan did not produce a particle indicating signal for the par ticle, the output of the shift register circuit 35 for the current scan of the particle is low causing the flip-flop circuit 51 to remain set. If the preceding operative scan of the particle produced a particle indicating signal for the particle, the output of the shift register circuit for the current scan of the particle is a particle indicating signal which is high causing the flip-flop to be reset to present a high signal to the input 56 of the NOR circuit 52. The trailing edge detector 50 produces a negative going pulse, as shown by signal pattern 6 of FIG. 3, corresponding to the trailing edge of a particle indicating signal 45 from the pulse stretcher circuit 19. Two low inputs will thus be present at the NOR circuit 52 when the flip-flop 51 has not been reset by the output of the shift register circuit 35 after the flip-flop 51 has been set by the leading edge detector 49 causing a high signal to appear at the output of the NOR circuit which is used to increase the count by one. A low input to the NOR circuit 52 from the trailing edge detector 50 plus a high input from the flip-flop 51 due to resetting of the flip-flop 51 by a high or particle indicating signal received from the shift register circuit 35 causes a low signal to be produced at the output of the NOR circuit 52 which will not effect a count. In summary, if a high or particle indicating signal from shift register circuit 35 needed to reset flip-flop 51 is applied after the flipflop 51 has been set by the leading edge detector 49 and before the trailing edge detector 50 provides a negative going pulse to the NOR circuit 52, the NOR circuit 52 will not provide a count signal, but if a high or particle indicating signal from shift register 35 needed to reset flip-flop 51 is not applied after the flip-flop 51 has been set by the leading edge detector and prior to the occurrence of a negative going pulse from the trailing edge detector 50, the NOR circuit 52 will provide a count signal. Accordingly, since a particle indicating signal will be presented to the flip-flop circuit 51 from the shift register circuit 35 prior to the end of the particle indicating signal obtained from circuit 19 for every operable scan of a particle subsequent to the first particle indicating signal scan, only one count signal will be produced for each particle giving rise to a particle indicating signal.

The ouput of NOR circuit 52 is applied to one input of a NAND circuit 58 which has its second input connected to a motor and count control circuit 59. The output of the NAND circuit 58 is applied to a count and display circuit 60. A high signal must be supplied to the NAND circuit 58 from the motor and count control circuit 59 when a count signal is supplied to the NAND circuit 58 from NOR circuit 52 to cause the NAND circuit 58 to provide a low signal to the count and display 60 to cause the count to be increased by one. The count and display 60 may be a readout tube arrangement which is well known to those skilled in the art. The motor and count control circuit also connects with the motor 27 to initiate movement of the field holder 12 by motor 27 for a scan. The motor 27 operation and application of the necessary signal to the NAND circuit 58 to permit passage of a count signal are correlated so counting can begin as soon as the field holder 12 has begun its movement for a scan.

As has been indicated, operation of the signal coincidence circuitry 48 is such that a positive signal presented on conductor 47 will cause a count signal to be presented at the output of NOR circuit 52 provided a positive or high signal is not received at the reset input 54 of flip-flop 51 prior to the termination of the positive signal presented on conductor 47. Using the connections to the circuitry 48 as indicated in FIG. 1 and FIG. 6, it was shown that only the first particle indicating signal for a scanned particle presented on conductor 47 from the pulse stretcher circuit 19 would cause a count signal to be produced with all subsequent particle indicating signals for a particle obtained from the pulse stretching circuit being ineffective to produce a count signal due to the positive reset signals presented by the shift register circuit 35 during all operable scans of a particle subsequent to the first operable scan.

It is possible to modify the circuitry of FIG. 1 slightly to provide alternate operable arrangement. The output on conductor 47 from pulse stretcher circuit 19 can be connected to the reset input 54 of flip-flop 51 while the output of the shift register circuit 35 is changed to connect to the leading edge detector 49 and the trailing edge detector 50. This will also provide a circuit which will operate so that only one count signal will be produced for each particle scanned that is large enough to produce a particle indicating signal. Referring to FIG. 3A, the signal pattern 4 is representative of the output from the shift register circuit 35 for an operative scan subsequent to the operative scan resulting in the initial particle indicating signal per 43 of FIG. 3. This signal applied to circuits 49 and 50 causes signal patterns 5a and 6a to be produced at the output of circuits 49 and 50 respectively. The signal pattern 45 of FIG. 3A is the signal pattern obtained from the pulse stretcher 19 which is now considered being applied to the reset input of flip-flop 51. With this arrangement, the pulse stretcher circuit 19 for a given particle scanned will provide a reset signal to the flip-flop 51 for each particle indicating signal received from the shift register circuit 35 except the last such signal received which will be due to the last particle indicating signal scan produced by a scan of the particle. The last particle indicating signal from the shift register circuit 35 for a given particle will be provided when the current scan is not giving rise to a particle indicating signal at the conductor 47 of the pulse stretcher circuit 19 causing such last particle indicating signal from circuit 35 to give rise to a count signal from NOR circuit 52 of the signal coincidence circuit 48.

No consideration has been given feedback circuit 20 which forms a part of the circuitry for processing the signals received from the photomultiplier l5 and it is not needed if only well-defined objects were to be counted. However, cases do arise where the light intensity at the photomultiplier 15 as an object is scanned varies due to the object itself plus the electrical noise from the photomultiplier, variations in the optical characteristics of the field holder 12 and variations in the intensity of the light source 23, causing erratic triggering of the minimum intensity and size circuits 17, 18. The feedback circuit 20 eliminates such erratic triggering by applying the output from the minimum size circuit 18 to the minimum intensity circuit 17 and minimum size circuit 18 to effect a reduction in the threshold requirements so that once a signal is developed at the output of the minimum size circuit 18 it will continue for the duration of the scan of the signal producing particle.

The signal from the output of the minimum size circuit 18 is applied to the feedback circuit 20 via a NOR circuit 61 since the output of the shift register circuit 35 is also applied to the feedback circuit 20. By using the NOR circuit 61, the prsence of a feedback signal from either circuit 18 or 35 will then effect the desired reduction in the threshold requirements. Inputs 62 and 63 of the NOR circuit 6! connect with the output of a minimum size circuit 18 and shift register circuit 35, respectively.

Since the clock pulse generator 11 provides the number of clock pulses to the shift register circuit 35 during each operative scan to complete one cycle of operation, the effective data input shifted by the shift register circuit 35, such as shown by signal pattern 46, will appear at the output of the shift register circuit 35 at the same point in the next corresponding operative scan.

However, one clock pulse is supplied to the shift register circuit 35 for each scan cycle in addition to the clock pulses provided by the clock pulse generator 11, causing the output of the shift register circuit 35 to be advanced one clock pulse each scan cycle. Signal pattern 4 of FIG. 3 therefor shows the output of the shift register circuit 35 presented during the operative scan which follows the operative scan which gave rise to the singal pattern 46 and as can be seen, is the same as signal pattern 46, but is advanced one clock pulse. This provides the feedback circuit with a signal from the shift register circuit 35 one clock pulse earlier causing reduction of the threshold levels for the minimum intensity circuit 17 and the minimum size circuit 18 by the feedback circuit due to the output of the shift register circuit 35 to occur earlier during corresponding operative scans subsequent to the first operative scan of a particle giving rise to a particle indicating signal for the particle thus assuring that a particle indicating signal will be provided for each corresponding operative scan of the particle subsequent to the first particle indicating operative scan. Having a particle indicating signal for each corresponding operative scan of a particle subsequent to the first particle indicating operative scan which will operate the shift register circuit 35 is important if the shift register circuit is to provide the necessary particle indicating signal at its output for operation of the signal coincidence circuitry 48 to prevent multiple counts of a single particle.

The advantage of advancing of the shift register circuit 35 output one clock pulse can be appreciated by considering the signal patterns of FIG. 3. If the next operative scan of a particle were to repeat the signal 42 of FIG. 3, the output of the shift register circuit 35, when advanced one clock pulse as shown per signal pattern 4, indicates the particle indicating signal portion of the shift register circuit 35 output (shaded portion) coincides with the photomultiplier output per signal pattern 42 to improve the desired signal coincidence needed to prevent a multiple count. The additional clock pulse needed to advance theshift register 35 is providedby using the synchronizing pulse provided by the mirror control circuit 22 at the beginning of each scan cycle. It is applied to the shift register 35 via the NAND circuit 37 which is connected with the mirror control circuit 22 by the conductor 64.

Since the scan provided by the oscillating mirror 21 and movement of the field 12 is sinusoidal in nature the rate of the scan will vary. Compensation for this inherent nonlinearity in the rate of scan is provided by applying a signal to the minimum size circuit 18 during each operational scan which is proportional to the magnitude of the rate of scan. Such a signal may be obtained from the mirror control circuit 22 since it controls the mirror 93 and receives information from the mirror 21 assembly relative to the position of mirror 93. The signal providing the compensation is applied to the minimum size circuit via the conductor 69.

The output of the photomultiplier 15 is a function of the intensity of the light from the background of the field being scanned and from the objects being counted. Many times there is only a slight difference between the two intensity levels making it essential the intensity threshold adjustment for the minimum intensity circuit 17 be accurately made for an accurate count of particles of a predetermined size range. This is made possible by that portion of the circuitry of FIG.

1 which includes the amplifier 70, the nonlinear transfer circuit 71 and the display device 72. The amplifier is connected to the output of the leading edge detector 49. The amplifier has its output applied to the nonlinear transfer circuit 71 which in turn drives the display device which can be a simple d.c. ammeter. This arrangement provides for a simple method to establish the optimum intensity threshold setting. The operator selects a representative portion of the field 12 for making the intensity threshold adjustment and moves the field 12 into position for scanning. During the scanning which is done only by the mirror 93, the intensity threshold adjustment is moved from one extreme end of its range to the other while the deflection provided by the ammeter 72 is observed. The meter 72 will initially have a low reading, then reach a maximum reading at some intermediate range and then drop off to a low reading. The operator need only manipulate the threshold adjustment as he observes the meter 72 until he is satisfied that the setting is about mid-range of the span providing the maximum meter reading. While the output from the leading edge detector 49 has been used, it is apparent that the output of the trailing edge detector 50, can be used to provide the same information required to provide the desired input for the display device 72. It is only necessary to invert the output from the trailing edge detector 50 to use it with circuits 7072.

The apparatus as shown in FIG. 1 uses an operative scan which includes only the first scan of each scan cycle. The arrangement per FIG. 1 can be easily modified so the operational scan includes both scans of a scan cycle. This requires a reduction of the frequency of oscillator 31 so the selected number of clock pulse begin with each scan cycle and continue until just prior to the end of a scan cycle. This, of course, means two count signals are produced when a particle is intercepted by both scans of a scan cycle. This requires the addition of a divide by two divider to the arrangement of FIG. 1 so the proper number of count signals are provided. Such a divider when used is connected between the output of the NAND circuit 58 and the count and display 60. I

The fact that the frequency of the oscillator 31 can be changed and no change need be made in the shift register circuit 35 makes it possible to use an oscillating mirror 21 that need not be manufactured to any close tolerances as far as its mechanically resonant frequency is concerned since any variation from unit to unit is taken care of by the oscillator 31 frequency adjustment. In addition, it can be appreciated that other scanning schemes having a scanning frequency different from that provided by the oscillating mirror 21 can be readily adapted for use with the electronics provided for the apparatus of FIG. 1.

SPECIFIC COMPONENTS .AND CIRCUITRY Some of the apparatus and its operation has been described largely in terms of the functions performed by the various elements and combinations thereof as set forth in FIG. 1. Further details regarding circuitry and components that can be used to facilitate construction of an apparatus as disclosed in connection with FIG. 1 will be given.

FIG. 5 sets forth the details for circuits which can be used for the clock pulse generator 11, mirror assembly 21, mirror control circuit 22, minimum intensity circuit 17, minimum size circuit 18 and the feedback circuit 20.

The oscillator 31 for pulse generator 11 includes a free-running multivibrator with provision made to connect a signal from flipflop 33 for inhibiting operation of the multivibrator. A pulse forming portion is also included for providing the narrow clock pulses required for operation of the pulse stretcher circuit 19 and the shift register 35. Adjustment of its frequency of operation is made possible by the potentiometer 74. The inhibiting signal from flip-flop 33 is applied to the multivibrator via an inverter circuit 76. The collector of transistor 86 is connected to an inverter 84 which in turn connects with the divider 32 and via conductor 13 to the pulse stretcher 19. The output of the inverter 84 is also applied to the pulse forming portion of the generator 11 which is a differentiating circuit that includes the capacitor 87 and resistor 88. The diode 89 connected in parallel with resistor 88 is used to provide positive voltage protection for the NAND circuit 37 to which the output from differentiating circuit of the oscillator 31 is applied.

A particle counter which uses 2 or 1024 clock pulses for an operative scan provides adequate resolution. The number of clock purles per operative scan establishes the requirements for divider 32 and the number of stages for shift registercircuit 35. With 1024 clock pulses used for an operative scan, the divider 32 is capable of dividing by 1024 and the shift register circuit 35 requires 1024 clock pulses for one complete cycle of operation.

A divider 32 capable of dividing by 1024 may be formed by connecting three hexadecimal counters 90-92, as shown in FIG. 5. The counters may be 7493N type integrated circuits. The actual terminal designations for the counters are shown. The first flip-flop circuit of counter 90 is not included as a part of the di vider 32 nor is the last flip-flop of counter 92. The last flip-flop of counter 92 can be used in the flip-flop circuit 33 which also includes an inverter 121 for receiving a reset signal. Upon receipt of the l024th pulse from oscillator 31, all of the flip-flops for the divider 32 will be-pr esenting a 0 at their outputs. As the final flipflop of the divider 32 presents a 0, the flip-flop circuit 33 will present a l at its output which via conductor 34 is applied to the oscillator 31 to inhibit its operation. The inhibit signal is removed when the inverter 121 of flip-flop circuit 33 receives a reset signal from the mirror control circuit 22 via conductor 30.

In the event the operative scan includes the entire scan cycle, the first flip-flop of counter 90 can provide the divide by two circuit which, as indicated earlier, is then required to be connected between the NAND circuit 58 and the count and display 60.

An oscillating mirror capable of oscillating at a rate of 200 cycles per second may be used. A Bulova American Time Products L50 scanner may be used as the oscillating mirror assembly 21. The L50 scanner includes the mirror 93 supported by a taut band at its center with permanent magnets 94 and 95 mounted at opposite ends of the mirror. The magnets 94 and 95 are magnetically coupled to coils 96 and 97, respectively, and are included as a part of the LS0 scanner. A capacitor 98 is connected in parallel with coil 97 to reduce high frequency noise pick-up. Coil 97 is used for sensing the position of the mirror 93 with respect to magnet 95 while coil 96 serves as drive coil and has a capacitor 9 connected in parallel with it to suppress noise generation. The drive and sensing coils 96, 97 are connected as a part of the mirror control circuit 22 which includes an oscillator-amplifier utilizing two operational amplifiers 99 and 100. Amplifier 99 receives a sinusoidal input from the sensing coil 97 via conductor 28 which is amplified and applied to the amplifier 100 connected to operate in the saturated mode. The output from amplifier 100 is essentially a square wave which is applied to the drive coil 96 via conductor 29. The amplifier oscillator serves to keep the oscillating mirror 21 oscillating at its mechanically resonant frequency which is approximately 200 HZ. The amplitude of the oscillations is determined by the setting of the potentiometer 112.

The mirror control circuit 22 also provides a signal for advancing the shift register 35 one clock pulse and for resetting the flip-flop 33 of the clock pulse generator 11. The circuitry to the right of the amplifier 100 in FIG. 5 is provided for this purpose. It sharpens the edges and then differentiates the output from amplifier 100 to obtain the narrow pulse needed at the start of each scan cycle to reset flip-flop 33 and advance the shift register circuit 35. As was the case for the differentiating circuit included with oscillator 31 circuitry, the diode connected across the resistor 119 of the differentiating circuit portion is used to provide positive voltage protection for the NAND circuit 37 and, also inverter 121 of the fiip-flop circuit 31.

Amplification of the output of the photomultiplier 15 can be obtained using any of a number of suitable amplifiers for the amplifier 16 which connects with the minimum intensity circuit 17. A circuit suitable for use as a minimum intensity circuit 17 is shown in Fig. 5 and includes means for adjusting the bias of an operational amplifier 3 which establishes the intensity threshold. The bias adjustment is obtained using the adjustable resistance formed by the fixed resistor 122 and a potentiometer 123 connected between a positive 5 volts and ground with the movable contact of the potentiometer connected via a switch 124 to either input of the amplifier 3. The switch 124 is provided since the particles being counted can'give rise to an increase in current at' the photomultiplier 15 or may cause a decrease in the output of the photomultiplier 15. The movable contacts of switch 124 are therefore positioned to the left or to the right dependent upon the type of particle being counted. If the signal is in excess of the bias setting, the minimum intensity threshold is satisfied by the detected particle causing a negative going pulse to be presented at the output of the amplifier 3. The duration of the pulse is determined by the length of the interception of the particle by the scan.

A circuit suitable for use as a minimum size circuit 18 is shown in FIG. 5 and includes an integrating circuit portion and a level detector circuit portion. The integrating circuit portion includes the operational amplifier 134 and transistor 142 while the level detector circuit portion includes the operational amplifier 147. The operational amplifier 134 has its input coupled to the output of the operational amplifier 99 of the mirror control circuit 22 and receives a sinusoidal signal when the mirror 93 is oscillating. The output of amplifier 134 at any instant is therefore proportional to the rate of scanning of mirror 21 and is negative since amplifier 134 acts as an inverter. The output of the amplifier 134 is coupled to the base of the transistor 142 to control the operation of the transistor 142 to provide compensation for the nonlinear rate of scan present when the disclosed sinusoidal scanning arrangement is used. The output of the minimum intensity circuit 17 is also connected to the base of transistor 142 and presents a negative signal indicative of the sensing of an object. During the half cycle corresponding to the operative scan, the transistor 142 circuitry integrates any signal received from the minimum intensity circuit 17 with the resulting signal being proportional to the duration and magnitude of the signal from the minimum intensity circuit 17 and the rateor scanning by the mirror 21. If the signal developed by the integrating process has an amplitude sufficient to overcome the bias setting of the operational amplifier 147 determined by the potentiometer 153 which establishes the size threshold, the operational amplifier 147 will provide an output signal having a duration corresponding to the time the integrated signal exceeds the signal level required by the bias setting. The output of the amplifier 147 is coupled to the pulse stretcher circuit 19 and to one input of the NOR circuit6l connecting with the feedback circuit 20.

The other half cycle of the sinusoidal signal from the motor control circuit 22 presented to amplifier 134 does not influence the operation of the amplifier 134 since it then is under the control of the positive voltage coupled to the'input of the operational amplifier via the resistor 131 connected in series with resistor 136 between a positive 15 volts and the base of transistor 142. The positive voltage then causes the output of the transistor 142 to stay low irrespective of a signal that may be received by the minimum circuit 17 from the minimum intensity circuit 18.

In the event the operative scan includes both scans in the scan cycle, the minimum size circuit 18 is modified slightly to provide the rate of scan compensating signal for the entire scan cycle. This requires the removal of the resistor 131 which connects with the positive 15 volts and replacing it with a diode having its cathode connected to the output of operational amplifier 99 and its anode connected to the anode of diode 135. Such a diode may be of the same type as is used for diode 135.

A circuit suitable for use as the feedback circuit is set forth at the bottom of FIG. 5 and includes an inverter 154 for amplifying and inverting the output from NOR circuit 61. Transistor 159 conducts in response to the output of the NOR circuit 61 when a feedback signal is received by the NOR circuit from the shift register circuit 35 or the minimum size circuit 18 causing the threshold requirements for the minimum intensity and size circuits 17 and 18 to be reduced.

A suitable shift register circuit 35 is shown in FIG. 6 together with details of circuits suitable for use in forming the signal coincidence circuitry 48. The shift register circuit 35 includes a 1024 bit static shift register 163 available under the designation 2533V with a resistor 164 connected between the clock input for the shift register 163 and a positive 5 volt source (not shown). The output of the shift register 163 is connected to an inverter 165 the output of which provides the output for the shift register circuit 35.

The leading edge circuit 49 and the trailing edge circuit 50 are identical except that the leading edge circuit includes an inverter 166 connected to the differentiating portion including the capacitor 167 and resistor to the flip-flop circuit 51. A diode 169 is connected across the resistor 168 to provide positive voltage protection for the inverter circuit 170. The trailing edge circuit has the differentiating circuit including capacitor 171, resistor 172 and the positive voltage protecting diode 173. With this arrangement, the positive going leading edge of a particle indicating signal received from the pulse stretcher 19 causes a positive going pulse to be produced by the leading edge circuit 49 to set the flip-flop 51 while the negative going trailing edge of the signal from pulse stretcher 19 causes a negative going pulse to be produced by the trailing edge circuit 50 which is applied to the input 57 of the NOR circuit 52.

The flip-flop circuit 51 may be formed using two NOR circuits 174 and 175. The set input for the circuit 51 is the input of NOR circuit 174 connected to the leading edge circuit 49. The other input of NOR circuit 174 is connected to the output of NOR circuit 175. The output of NOR circuit 174 connects to one input of NOR circuit 175 and to input 56 for NOR circuit 52. The other input for NOR circuit 175 is the reset input 54 for flip-flop 51 and is connected to the output of the shift register circuit 35.

The circuitry which may be used for the amplifier 70 and the non-linear transfer circuit 71 to drive a display device 72, such as a dc. ammeter, in accordance with the positive going pulses at the output of the leading edge circuit is also shown in FIG. 6. The transistor 177 provides the necessary amplification for amplifier 70 while the capacitor 179 and resistors 178, 180 provide the transfer circuit 71 connected to drive the meter 72.

The specific circuits shown in FIGS. 4, 5 and 6 are purely exemplary and other equivalent circuits may be utilized. To complete the disclosure for the circuits of FIGS. 4, 5 and 6, the various circuit components which have not been discussed previously with respect to value or type, are listed below with the nominal values or type listed for each component. It should be noted also that the dc. voltages required for operation of some of the components have not been indicated. A positive 5 volts is required for operation of the NAND, NOR inverter, flip-flop and counter circuits used. The operational amplifiers connect with a positive 15 volts and a negative 12 volts.

VALUE OR TYPE -Contmued COMPONENT VALUE OR TYPE 74, 112 500 ohm 123, 153 1K ohm 101 K ohm Resistors 139 33 ohm 178 100 ohm 103 390 ohm 77. 81 470 ohm 75 560 ohm 7 820 ohm 107, 113. 125,145. 149, 155 1K ohm 78, 106. 111,156, 176, 181 1.5K ohm 88. 117, 119, 148,152 164,168,172 2.2K ohm 114. 122,130,141, 180 3.3K ohm 144 4 7K ohm 132, 133 4.99K ohm 127 5.6K ohm 105, 131. 157,158 10K ohm 79. 80, 108, 136 K ohm 143 33K ohm 102 47K ohm 146 120K ohm 110.126 lMegohm What is claimed is:

1. Particle detection apparatus for determining the number of those particles having a predetermined size that are present in a pluralityv of particles dispersed over a field comprising:

scanning means for scanning said field using a succession of scan cycles, each of said scan cycles moving across said field for successive scanning of each particle presented to the scan cycle, said scanning means providing a particle indicating signal each time a particle having a predetermined size is scanned and supplying a synchronizing pulse at the beginning of each scan cycle; clock pulse generator connected to said scanning means for receiving said synchronizing pulse and supplying a selected number of clock pulses in response to said synchronizing pulse and at a rate causing the last of said clock pulses to occur prior to completion of a scan cycle;

a scan cycle memory including a shift register having a clock input, a data input and an output, said shift register requiring said selected number of clock pulses for a cycle of operation, said clock input connected to receive said clock pulses, said data input connected to serially receive each of said particle indicating signals developed by said scanning means, said shift register entering a particle indicating signal when present at said data input while one of said clock pulses is present at said clock input and shifting said entered particle indicating signal to said output upon subsequent receipt by said clock input of clock pulses equal to said selected number; and

a signal coincidence circuit for presenting a count signal in response to a first signal unless a second signal is received by said signal coincidence circuit during said first signal, said signal coincidence circuit connected to said scanning means to receive said particle indicating signals from said scanning means as one of said first and second signals and connected to said output of said shift register to receive said particle indicating signals from said shift register output as the other of said first and second signals.

2. A particle detection apparatus in accordance with claim 1 wherein said one of said first and second signals is said first signal and said other of said first and second signals is said second signal.

3. A particle detection apparatus in accordance with claim 1 wherein said one of said first and second signals is said second signal and said other of said first and second signals is said first signal.

4. A particle detection apparatus in accordance with claim 1 wherein said scan cycle includes a scan in one direction across the field and a scan in the opposite direction across the field with the last one of said selected number of clock pulses for a scan cycle produced prior to the start of said scan in said opposite direction across the field.

5. A particle detection apparatus in accordance with claim 1 wherein said scan cycle includes a scan in one direction across the field and a scan in the opposite direction across the field with the last one of said selected number of clock pulses for a scan cycle produced shortly beforecompletion of said scan in said opposite direction across the field, said apparatus further including a-divider circuit connected to said signal coincidence circuit for providing one count signal for every two of said count signals produced by said signal coincidence circuit.

6. A particle detection apparatus in accordance with claim 1 wherein the frequency of said scan cycles rela tive to the rate at which said clock pulses are produced is adjustable.

7. A particle detection apparatus in accordance with claim 1 wherein said clock pulse generator includes means for adjusting the rate at which said clock pulses are produced to establish the frequency of the scan cycles'relative to the rate said clock pulses are produced for determining the point in a scan cycle when the last one of said selected numberof clock pulses following receipt of a synchronizing pulse is provided.

8. A particle detection apparatus in accordance with claim 1 wherein said scanning means includes a light responsive device;

a source of light;

means establishing an optical path between said light responsive device and said light source with the field positioned in said optical path including an oscillating mirror disposed between said field and the light responsive device for scanning back and forth across the field in one direction;

a mirror control circuit having means operatively coupled to said mirror to sense the position of said mirror and drive said mirror for oscillation at a selected frequency and means for moving the field transversely to the movement of said mirror whereby a sinusoidal optical scan of the field is presented to said light responsive device when the mirror is oscillating in said one direction while said field is moved transverse to said one direction.

9. A particle detection apparatus in accordance with claim 8 wherein said mirror control circuit includes a pulse forming circuit connected to said clock pulse generator for providing a pulse at the beginning of each scan cycle as the synchronizing pulse for said clock pulse generator.

10. A particle detection apparatus according to claim 1 wherein said scanning means includes circuitry for establishing the minimum light intensity that a particle must present when scanned before said particle indicating signal can be produced; and a feedback circuit connected between the output and input of said circuitry for reducing the minimum light intensity requirement for the scan portion remaining for a particle being scanned once said particle indicating signal for such particle is initiated.

11. A particle detection apparatus according to claim 10 wherein the output of said shift register is connected to said feedback circuit.

12. A particle detection apparatus according to claim 1 wherein said scanning means includes circuitry for establishing the minimum size that a particle must present to a scan before said particle indicating signal can be produced; and a feedback circuit connected between the output and input of said circuitry for reducing the minimum size requirement for the scan portion remaining for a particlebeing scanned once said particle indicating signal for such particle is initiated.

13. A particle detection apparatus according to claim 12 wherein the output of said shift register is connected to said feedback circuit.

14. A particle detection apparatus according to claim 1 wherein said scanning means includes circuitry having a minimum intensity circuit for establishing the,

light intensity that a particle must present when scanned before said particle indicating signal can be produced and a minimum size circuit for establishing the minimum width of a particle that must be scanned before said particle indicating signal can be produced; and a feedback circuit connected between the output of said circuitry and the input of said minimum intensity circuit back minimum size circuit for reducing the intensity and size requirements for the scan portion remaining for a particle being scanned once said particle indicating signal for such particle is initiated.

15. A particle detection apparatus according to claim 14 wherein the output of said shift register is connected to said feedback circuit.

16. A particle detection apparatus according to claim 1 wherein said scanning means includes a feedback circuit with the output of said shift register connected to the input of said feedback circuit and said synchronizing pulse provided by said scanning means is applied as a clock pulse to said shift register to shift the shift register one clock pulse causing the output of said shift register to be applied to said feedback circuit earlier in each scan subsequent to that first scan of said field.

l7. Particle detection apparatus for determining the number and size of particles present in a plurality of particles dispersed over a field including:

scanning means for scanning the field using a succession of scan cycles wherein each scan cycle proceeds across the field at a non-linear rate, said scanning means providing a signal each time a particle in the field is scanned, said signal having a duration dependent on the rate of scan and the width of the particle presented to the scan;

said scanning means including a control circuit for providing a signal having a non-linear rate corresponding to the non-linear rate of the scan cycle; signal processing circuitry connected to said scanning means for receiving said first-mentioned signal to provide a particle indicating signal when said first-mentioned signal has a predetermined amplitude and duration;

said signal processing circuitry including a minimum size circuit having an integrating circuit portion for producing a signal indicative of the duration of said tirst-mentioned signal, said integrating circuit portion connected to said control circuit for receiving said signal provided by said control circuit for controlling the operation of said integrating circuit whereby said signal indicative of the duration of said first-mentioned signal provided by said integrating circuit portion is proportional to the nonlinear rate of each scan cycle.

18. Particle detection apparatus, which scans a field containing a plurality of particles and produces particle indicating signals in accordance with the size of the particles scanned and the light intensity received from the particles scanned relative to an adjustable intensity threshold, comprising:

means for enabling scanning to occur back and forth on a line across an operator selected portion of the field causing particle indicating signals to be produced;

means connected to said first-mentioned means for providing a pulse signal for each particle indicating signal produced when scanning said operator selected portion;

means connected to said second-mentioned'means,

including a display means responsive to said pulse signals for providing a single visual indication which varies in proportion to the number of said pulse signals received whereby the intensity threshold can be varied until said single visual indication indicative of the receipt of the maximum number of said pulse signals is established.

19. A particle detection apparatus according to claim 18 wherein said second mentioned means is a circuit responsive to the leading edge of the particle indicating signals.

20. A particle detection apparatus according to claim 18 wherein said second mentioned means is a circuit responsive to the trailing edge of the particle indicating signals.

21. A particle detection apparatus according to claim 18 wherein said last mentioned means includes an ammeter.

22. Particle detection apparatus for determining the number of those particles having a predetermined size that are present in a plurality of particles dispersed over a field comprising scanning means for scanning said field in a succession of scan cycles and including circuitry for establishing the minimum size that a particle must present before a particle indicating signal can be produced and a feedback circuit connected between the input and output of said circuitry for reducing the minimum size requirement for the scan portion remaining for a particle being scanned once said particle indicating signal for such particle is initiated.

23. Particle detection apparatus for determining the number of those particles having a predetermined size that are present in a plurality of particles dispersed over a field comprising scanning means for scanning said field in a succession of scan cycles and including circuitry for establishing the minimum light intensity that a particle must present before a particle indicating signal can be produced and a feedback circuit connected between the input and output of said circuitry for re- 21 ducing the minimum light intensity requirement for the scan portion remaining for a particle being scanned once said particle indicating signal for such particle is initiated.

24. Particle detection apparatus for determining the number of those particles having a predetermined size that are present in a plurality of particles dispersed over a field comprising scanning means for scanning said field in a succession of scan cycles and including circuitry having a minimum intensity circuit for establishing the light intensity that a particle must present when scanned before a particle indicating signal can be produced and a minimum size circuit for establishing the minimum width of a particle that must be scanned before said particle indicating signal can be produced; and a feedback circuit connected between the output of said circuitry and the input of said minimum intensity circuit and siad minimum size circuit for reducing the intensity and size requirements for the scan portion remaining for a particle once said particle indicating signal for such particle is initiated.

25. A particle detection apparatus according to claim 24 wherein said apparatus includes a scan cycle memory having its input connected for receiving said particle indicating signals and having its output connected to said feedback circuit, the output of said memory being indicative of the particle indicating signals produced during the preceding scan cycle.

26. A particle detection apparatus according to claim 25 wherein said scan cycle memory includes a shift reg- UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3, 867,613

DATED February 18, 1975 |NVENTOR(S) DAVID J. SCHOO'N it is certified that error appears in the ab0ve-identified patent and that said Letters Patent are hereby corrected as shown betow:

Column 3, line 19, change "changnes" to changes Column l, line 26, change "ouput" to output Column 5, line 61, insert "a" before "scan cycle".

Column 6, line t t, change "circut" to circuit Column 6, line 63, change "appled" to applied 7 Column 7, line 49, change "incnrease" to increase Column 7, line 63, omit "scann" and insert "of" after "scan".

Column 8, line t, change "firs" to first Column 9, line 30, change "ouput" to output Column 10, line 55, change "prsence" to presence Column 11, line 6, change "therefor" to therefore Column 11, line 9, change "singal" to signal Column 13, line 25, change "purles" to pulses Column 19, line 36, change "back" to and Column 22, line 2, change "siad" to said [sure] A Has 1:

RUTH C. MASON .t rresrr'ng Officer Signed and Sealed this twenty-fifth D ay 0f November 19 75 C. MARSHALL DANN ('mnmrls'sr'mrr'r 01' Parents and Trademurkzr 

1. Particle detection apparatus for determining the number of those particles having a predetermined size that are present in a plurality of particles dispersed over a field comprising: scanning means for scanning said field using a succession of scan cycles, each of said scan cycles moving across said field for successive scanning of each particle presented to the scan cycle, said scanning means providing a particle indicating signal each time a particle having a predetermined size is scanned and supplying a synchronizing pulse at the beginning of each scan cycle; a clock pulse generator connected to said scanning means for receiving said synchronizing pulse and supplying a selected number of clock pulses in response to said synchronizing pulse and at a rate causing the last of said clock pulses to occur prior to completion of a scan cycle; a scan cycle memory including a shift register havinG a clock input, a data input and an output, said shift register requiring said selected number of clock pulses for a cycle of operation, said clock input connected to receive said clock pulses, said data input connected to serially receive each of said particle indicating signals developed by said scanning means, said shift register entering a particle indicating signal when present at said data input while one of said clock pulses is present at said clock input and shifting said entered particle indicating signal to said output upon subsequent receipt by said clock input of clock pulses equal to said selected number; and a signal coincidence circuit for presenting a count signal in response to a first signal unless a second signal is received by said signal coincidence circuit during said first signal, said signal coincidence circuit connected to said scanning means to receive said particle indicating signals from said scanning means as one of said first and second signals and connected to said output of said shift register to receive said particle indicating signals from said shift register output as the other of said first and second signals.
 2. A particle detection apparatus in accordance with claim 1 wherein said one of said first and second signals is said first signal and said other of said first and second signals is said second signal.
 3. A particle detection apparatus in accordance with claim 1 wherein said one of said first and second signals is said second signal and said other of said first and second signals is said first signal.
 4. A particle detection apparatus in accordance with claim 1 wherein said scan cycle includes a scan in one direction across the field and a scan in the opposite direction across the field with the last one of said selected number of clock pulses for a scan cycle produced prior to the start of said scan in said opposite direction across the field.
 5. A particle detection apparatus in accordance with claim 1 wherein said scan cycle includes a scan in one direction across the field and a scan in the opposite direction across the field with the last one of said selected number of clock pulses for a scan cycle produced shortly before completion of said scan in said opposite direction across the field, said apparatus further including a divider circuit connected to said signal coincidence circuit for providing one count signal for every two of said count signals produced by said signal coincidence circuit.
 6. A particle detection apparatus in accordance with claim 1 wherein the frequency of said scan cycles relative to the rate at which said clock pulses are produced is adjustable.
 7. A particle detection apparatus in accordance with claim 1 wherein said clock pulse generator includes means for adjusting the rate at which said clock pulses are produced to establish the frequency of the scan cycles relative to the rate said clock pulses are produced for determining the point in a scan cycle when the last one of said selected number of clock pulses following receipt of a synchronizing pulse is provided.
 8. A particle detection apparatus in accordance with claim 1 wherein said scanning means includes a light responsive device; a source of light; means establishing an optical path between said light responsive device and said light source with the field positioned in said optical path including an oscillating mirror disposed between said field and the light responsive device for scanning back and forth across the field in one direction; a mirror control circuit having means operatively coupled to said mirror to sense the position of said mirror and drive said mirror for oscillation at a selected frequency and means for moving the field transversely to the movement of said mirror whereby a sinusoidal optical scan of the field is presented to said light responsive device when the mirror is oscillating in said one direction while said field is moved transverse to said one direction.
 9. A particle detection apparatus in accordance with claim 8 wherein said mirror control circuit includes a pulse forming circuit connected to said clock pulse generator for providing a pulse at the beginning of each scan cycle as the synchronizing pulse for said clock pulse generator.
 10. A particle detection apparatus according to claim 1 wherein said scanning means includes circuitry for establishing the minimum light intensity that a particle must present when scanned before said particle indicating signal can be produced; and a feedback circuit connected between the output and input of said circuitry for reducing the minimum light intensity requirement for the scan portion remaining for a particle being scanned once said particle indicating signal for such particle is initiated.
 11. A particle detection apparatus according to claim 10 wherein the output of said shift register is connected to said feedback circuit.
 12. A particle detection apparatus according to claim 1 wherein said scanning means includes circuitry for establishing the minimum size that a particle must present to a scan before said particle indicating signal can be produced; and a feedback circuit connected between the output and input of said circuitry for reducing the minimum size requirement for the scan portion remaining for a particle being scanned once said particle indicating signal for such particle is initiated.
 13. A particle detection apparatus according to claim 12 wherein the output of said shift register is connected to said feedback circuit.
 14. A particle detection apparatus according to claim 1 wherein said scanning means includes circuitry having a minimum intensity circuit for establishing the light intensity that a particle must present when scanned before said particle indicating signal can be produced and a minimum size circuit for establishing the minimum width of a particle that must be scanned before said particle indicating signal can be produced; and a feedback circuit connected between the output of said circuitry and the input of said minimum intensity circuit back minimum size circuit for reducing the intensity and size requirements for the scan portion remaining for a particle being scanned once said particle indicating signal for such particle is initiated.
 15. A particle detection apparatus according to claim 14 wherein the output of said shift register is connected to said feedback circuit.
 16. A particle detection apparatus according to claim 1 wherein said scanning means includes a feedback circuit with the output of said shift register connected to the input of said feedback circuit and said synchronizing pulse provided by said scanning means is applied as a clock pulse to said shift register to shift the shift register one clock pulse causing the output of said shift register to be applied to said feedback circuit earlier in each scan subsequent to that first scan of said field.
 17. Particle detection apparatus for determining the number and size of particles present in a plurality of particles dispersed over a field including: scanning means for scanning the field using a succession of scan cycles wherein each scan cycle proceeds across the field at a non-linear rate, said scanning means providing a signal each time a particle in the field is scanned, said signal having a duration dependent on the rate of scan and the width of the particle presented to the scan; said scanning means including a control circuit for providing a signal having a non-linear rate corresponding to the non-linear rate of the scan cycle; signal processing circuitry connected to said scanning means for receiving said first-mentioned signal to provide a particle indicating signal when said first-mentioned signal has a predetermined amplitude and duration; said signal processing circuitry including a minimum size circuit having an integrating circuit portion for producing a signal indicative of the duration of said first-mentioned signal, said integratiNg circuit portion connected to said control circuit for receiving said signal provided by said control circuit for controlling the operation of said integrating circuit whereby said signal indicative of the duration of said first-mentioned signal provided by said integrating circuit portion is proportional to the non-linear rate of each scan cycle.
 18. Particle detection apparatus, which scans a field containing a plurality of particles and produces particle indicating signals in accordance with the size of the particles scanned and the light intensity received from the particles scanned relative to an adjustable intensity threshold, comprising: means for enabling scanning to occur back and forth on a line across an operator selected portion of the field causing particle indicating signals to be produced; means connected to said first-mentioned means for providing a pulse signal for each particle indicating signal produced when scanning said operator selected portion; means connected to said second-mentioned means, including a display means responsive to said pulse signals for providing a single visual indication which varies in proportion to the number of said pulse signals received whereby the intensity threshold can be varied until said single visual indication indicative of the receipt of the maximum number of said pulse signals is established.
 19. A particle detection apparatus according to claim 18 wherein said second mentioned means is a circuit responsive to the leading edge of the particle indicating signals.
 20. A particle detection apparatus according to claim 18 wherein said second mentioned means is a circuit responsive to the trailing edge of the particle indicating signals.
 21. A particle detection apparatus according to claim 18 wherein said last mentioned means includes an ammeter.
 22. Particle detection apparatus for determining the number of those particles having a predetermined size that are present in a plurality of particles dispersed over a field comprising scanning means for scanning said field in a succession of scan cycles and including circuitry for establishing the minimum size that a particle must present before a particle indicating signal can be produced and a feedback circuit connected between the input and output of said circuitry for reducing the minimum size requirement for the scan portion remaining for a particle being scanned once said particle indicating signal for such particle is initiated.
 23. Particle detection apparatus for determining the number of those particles having a predetermined size that are present in a plurality of particles dispersed over a field comprising scanning means for scanning said field in a succession of scan cycles and including circuitry for establishing the minimum light intensity that a particle must present before a particle indicating signal can be produced and a feedback circuit connected between the input and output of said circuitry for reducing the minimum light intensity requirement for the scan portion remaining for a particle being scanned once said particle indicating signal for such particle is initiated.
 24. Particle detection apparatus for determining the number of those particles having a predetermined size that are present in a plurality of particles dispersed over a field comprising scanning means for scanning said field in a succession of scan cycles and including circuitry having a minimum intensity circuit for establishing the light intensity that a particle must present when scanned before a particle indicating signal can be produced and a minimum size circuit for establishing the minimum width of a particle that must be scanned before said particle indicating signal can be produced; and a feedback circuit connected between the output of said circuitry and the input of said minimum intensity circuit and siad minimum size circuit for reducing the intensity and size requirements for the scan portion remaining for a particle once said particle indicating sigNal for such particle is initiated.
 25. A particle detection apparatus according to claim 24 wherein said apparatus includes a scan cycle memory having its input connected for receiving said particle indicating signals and having its output connected to said feedback circuit, the output of said memory being indicative of the particle indicating signals produced during the preceding scan cycle.
 26. A particle detection apparatus according to claim 25 wherein said scan cycle memory includes a shift register. 